I saw a trick that is used in Bryce that could be something.
The idea was to duplicate the ocean primitive and adjust the height so it just pops through at the highest point (make primitive higher and lower the whole thing). Then shade with a nifty foam-like shader (I imagine 'cells' in the bump map).

As I am typing this, I realise that this system only works if there is no transparancy.
The workaround is of course to render the foam in a seperate version, where the ocean is 100% black, and then put the foam render on top of the render in Photoshop, with the blending mode set to 'lighten'.
But then you should have Photoshop...

Another way I can imagine is Boolean substraction, but I have no idea if that works with the ocean primitive, and if it brings a computer to a screeching stop...

Hope this helps. Not sure though... :-)